Messenger RNA regulation in human diploid fibroblasts

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Abstract
In resting, non‐growing human diploid fibroblasts the amount of rRNA is reduced 1.8‐fold, cytoplasmic polysomes are disaggregated, and the level of poly‐A RNA (mRNA) is reduced 1.8‐fold in relation to growing cells. The distribution of poly‐A RNA is altered in resting, non‐growing cells so that an average of 64% of the total cytoplasmic poly‐A RNA sediments along with particles lighter than 80S (prepolysomal) in sucrose density gradients. By comparison, in growing cells only 30% of the cytoplasmic poly‐A RNA sediments in the prepolysomal region. In SDS sucrose gradients, the sedimentation profile of the prepolysomal poly‐A RNA from resting cells resembles that of polysomal poly‐A RNA from those cells. In contrast, the average size of prepolysomal poly‐A RNA from growing cells is much smaller than that of the polysomal poly‐A RNA from those cells. These data are compatible with the possibility that resting cell prepolysomal poly‐A is untranslated mRNA. Also consistent with this interpretation are experiments which demonstrate that one‐quarter to one‐third of the prepolysomal poly‐A RNA of resting cells is recruited into polysomes in the presence of cycloheximide.

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